Page 291 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 225

and a pair of stiffeners should be provided opposite the tension flange when the
thickness of the column flange t is less than
f
P bf
t f 0.4 (9.64)
BF yc
Stiffeners required by the preceding equations should comply with the follow-
ing additional criteria:
1. The width of each stiffener plus half the thickness of the column web should
not be less than one-third the width of the flange or moment-connection plate
delivering the concentrated force.
2. The thickness of stiffeners should not be less than t /2.
b
3. The weld-joining stiffeners to the column web must be sized to carry the force
in the stiffener caused by unbalanced moments on opposite sides of the column.

FASTENERS IN BUILDINGS

The AISC specification for allowable stresses for buildings specifies allowable
unit tension and shear stresses on the cross-sectional area on the unthreaded
body area of bolts and threaded parts. (Generally, rivets should not be used in
direct tension.) When wind or seismic loads are combined with gravity loads,
the allowable stresses may be increased by one-third.
Most building construction is done with bearing-type connections. Allowable
bearing stresses apply to both bearing-type and slip-critical connections. In build-
ings, the allowable bearing stress F , ksi (MPa), on projected area of fasteners is
p
(9.65)
F p 1.2F u
where F is the tensile strength of the connected part, ksi (MPa). Distance mea-
u
sured in the line of force to the nearest edge of the connected part (end distance)
should be at least 1.5d, where d is the fastener diameter. The center-to-center
spacing of fasteners should be at least 3d.

COMPOSITE CONSTRUCTION

In composite construction, steel beams and a concrete slab are connected so
that they act together to resist the load on the beam. The slab, in effect, serves
as a cover plate. As a result, a lighter steel section may be used.

Construction in Buildings

There are two basic methods of composite construction.
Method 1 The steel beam is entirely encased in the concrete. Composite action
in this case depends on the steel-concrete bond alone. Because the beam is